1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping or engaging mode) such that an actual amount of slip (slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller of the power transmitting device eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle while minimizing the power loss due to slipping of the lock-up clutch during acceleration of the vehicle. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaged state. These fully releasing, fully engaging and slip control areas are defined by parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition. In a motor vehicle equipped with a fuel-cut device adapted to cut a supply of a fuel to the engine while the engine speed is higher than a predetermined fuel-cut speed, it is proposed to control the lock-up clutch in the slip control mode even during deceleration of the vehicle, in order to raise the engine speed and thereby increase the time duration during which the fuel supply to the engine is cut.
While the lock-up clutch is controlled in the slip control mode, the friction condition of the lock-up clutch may be unstable due to deterioration of a working oil, use of an unsuitable oil, or due to changing of the condition of the friction surface of the lock-up clutch. Consequently, the lock-up clutch may suffer from a phenomenon of so-called "judder", namely, random variation or fluctuation of the slip speed (amount of slip) of the lock-up clutch. Once the "judder" phenomenon happens, it tends to continue throughout the slip control operation, leading to deterioration of the running stability of the vehicle. In view of this drawback, it is proposed to terminate the slip control of the lock-up clutch in the event of detection of the "judder" phenomenon of the lock-up clutch, as disclosed in JP-A-60-151457.
However, the slip control apparatus for the lock-up clutch as disclosed in the publication identified above is adapted to terminate the slip control upon detection of the "judder", irrespective of the cause for the judder. This arrangement leads to a drawback that the slip control of the lock-up clutch does not provide an intended or desired effect.